Air conditioning systems are routinely employed within automobiles and other vehicles for creating comfortable conditions within the passenger compartment for the vehicle occupants. At outside temperatures above about 70.degree. F., it is difficult to maintain a comfortable passenger compartment temperature without first cooling the air that is being blown into the passenger compartment.
Typically, cooling of the air is accomplished by first compressing an appropriate refrigerant, such as the fluorocarbon known as Freon or other alternative refrigerants. Within an automobile, the engine-driven compressor compresses the vaporized refrigerant, thereby significantly raising the temperature of the refrigerant. The refrigerant then flows into a condenser where it is cooled and returned to its liquid state; thus, the heat added to the refrigerant in the compressor is transferred out of the system. The cooled liquid refrigerant is then sprayed through an expansion valve into an evaporator where it is again vaporized. The heat of vaporization required for vaporizing the refrigerant is drawn from the incoming outside air, which is blown around the evaporator. Any excess humidity contained within the incoming air is removed as condensation on the evaporator, therefore also drying the incoming air. The cooled, dry air then enters the passenger compartment of the vehicle.
So as to maximize the amount of surface area available to the incoming air and, correspondingly, to more efficiently cool and dry that air, the design of the evaporator unit is typically a tubeplate-and-air center type heat exchanger containing flat ribbed tubes, similar to the tube-and-fin type designs. The evaporator is assembled by stacking and brazing together matching clad aluminum sheet components. Each of the clad aluminum components is formed from special aluminum brazing stock material which has been stamped so as to construct an internal arrangement of tubular shapes. The matching stamped aluminum tubeplates are then stacked and brazed together in a vacuum furnace, so as to form the tubeplate-and-center type evaporator.
As stated, the evaporator contains a multitude of internal tubes, which all must be brazed during a single brazing operation. There are practically hundreds of these brazements which must be formed concurrently. Generally, this is accomplished by employing a particular aluminum alloy brazing stock material as the matched tubeplates. The aluminum alloy brazing stock material consists, for example, of an appropriate aluminum alloy core which has been clad on both sides with an aluminum-based brazing alloy. Typically, the cladding layers are an aluminum-silicon eutectic brazing alloy characterized by a melting point lower than the core aluminum alloy. Therefore, the clad layers of brazing alloy melt during the vacuum brazing operation and flow toward the desired joint regions and upon cooling solidify to form the brazements. The core aluminum alloy does not melt during the brazing operation and thereby constitutes the structural part of the tubeplate-and-center type evaporator. In the past, the aluminum alloy brazing stock material which has been routinely used to form these types of plate type evaporators consists of a core layer of aluminum alloy AA 3005, as designated by the Aluminum Association (AA), that has been clad on both sides by an aluminum-silicon brazing alloy, specifically aluminum alloy AA 4047.
Generally speaking, this particular aluminum alloy brazing stock material has performed satisfactorily over the years. The material is easily stamped for formation of the tubeplates. In addition, there are no inherent brazing difficulties associated with the use of this particular brazing stock material, and the structural integrity of the material during use is sufficient.
However, the corrosion resistance of evaporator units formed from this specific material is less than desired. In particular, it has been found that if the evaporator is positioned horizontally within the engine compartment rather than vertically, so as to allow the refrigerant to settle within the tubular structures when the unit is not in operation, there is an increased incidence of failures due to corrosion of the evaporator unit. This is especially problematic if the horizontally-oriented tubeplates of the evaporator unit are also exposed to long periods of wetness due to the use of a climate control switch within the passenger compartment of the automobile which automatically regulates the temperature of the incoming air.
The corrosion problem associated with this material appears to be due, not only to the presence of the refrigerant which tends to chemically attack to aluminum, but also the presence of sulfur within the incoming air. A high concentration of sulfur tends to generally occur around the corrosive perforations of the evaporator and the perforations tend to occur in those aluminum tubeplates which face towards the incoming air from outside of the vehicle. It is believed that sulfur is derived from either sulfuric acid droplets and/or sulfur dioxide. Airborne sulfur and/or sulfuric acid droplets could arise from the smokestacks of power utilities burning high sulfur coal or from the catalytic converters of automobiles which may also be the source for the sulfur dioxide gas.
In the past, the corrosion resistance of an air conditioning evaporator formed from this conventional aluminum brazing stock material would typically be enhanced by coating the evaporator with a protective chromate layer. However, due to increased concerns about the environment, the industry is working to eliminate the use of this protective chromate coating because of the toxic by-products produced by the process. The goal is to eliminate the use of the chromate conversion process entirely.
Therefore the industry needs an aluminum alloy brazing stock material which has improved corrosion resistance so that an evaporator unit formed from this material can survive the aggressive automotive environment without the requirement for an additional protective chromate coating. In addition, the core alloy of this brazing stock material must be capable of resisting corrosion due to the compositionally different braze alloy present at the multitude of brazed joints within the evaporator. Lastly, the core alloy of this improved brazing stock material must also be sufficiently ductile so as to respond to the room temperature stamping operations used for formation of the multitudes of internal tubes, while having sufficient strength to contain the high pressure refrigerant over repeated thermal cycling within a typical automotive environment.